Double-ground plane attenuator



June 28, 1960 E. A. DoRsETT 2,943,283

DOUBLE-GROUND PLANE ATTENUATOR BW ffm-,

A TTORNE YS June 28, 1960 E. A. DoRsE-rT DOUBLE-GROUND PLANE ATTENUATOR 2 Sheets-Sheet 2 Filed June 10, 1957 l n v N INVENTOR. EDWARD DORSETT BY *'yim/ A Tron/v5 YS United States Patent O i DOUBLE-GROUND PLANE A'ITENUATOR Edward A. Dorsett, Melbourne, Fla., assignor to Radiation, Inc., Melbourne, Fla., a corporation of Florida Filed June 10, 1957, Ser. No. 664,815

8 Claims. (Cl. 333-81) The present invention relates to liquid-cooled attenuators of high frequency electrical energy and more par ticularly to a liquid-cooled, double-grouud-plane transmission line attenuator for employment with coaxial cables.

An attenuator of electrical energy employed to' dissipate appreciable power must utilize resistive elements that yare either large in size or, if small, are cooled by artificial means such as forced air or forced liquid flow. In order to build a practical attenuator the resistive elements thereof must be considered as lumped constant elements rather than as distributed parameter elements, which requires that the resistive elements be physically small compared to the wave length of the operating frequency of the system in which the attenuator is employed.

The present invention is concerned with an attenuator of considerable power handling capability to be employed in systems operating in the R.F.y range of frequencies, which are normally transmitted via coaxial cables. At the frequencies involved the resistive dissipative elements of the attenuator must be relatively small to be considered as lumped constants and since the invention is concerned with dissipation of considerable amounts of power, artificial cooling o'f the resistive elements is mandatory.

The prior art attenuators for utilization at radio fre- `quencies comprises a coaxial cable having a resistive center conductor cooled by a moving liquid dielectric, such as oil, which conveyed the heat of the center condueto'r to the kouter conductor and through the outer conductor to the surrounding air. Considerable ditiiculty is encountered in obtaining a flow of `coolant through a `coaxial cable that is adequate to achieve the desired cooling of the centered conductor. Further the means for effecting movement of the coolant must be disposed externally of the cable and therefore entry and exit connection for the coolant must be made in the outer conductor of the coaxial Acable which produces undesired alterations in the electrical transmission characteristics of the cable.

In accordance with the present invention a double* ground-plane transmission line having resistive elements is employed as an attenuator for R.F. energy. A doublelground-plane transmission line comprises two wide parallel conductors having a center conductor disposed therebetween. The width of the center conductor issufiiciently small, compared to the width of the two parallel conductors, that the parallel conductors appear infinite and R.F. leakage at the open edges of the line is minimized. Since the sides of a double-ground-plane line may be open, a cooling liquid may be freely circulated over the resistive elements and the circulating equipment may be lpositioned externally of the walls of the line so as to have no effect upon the transmission properties of the line. The parallel planes of a double-ground-plane line are both grounded and may be connected together both electrically and physically by conductive walls `parallel to the center conductor, so long as the walls are displaced 2,943,283 Patented June 2s, 1960 ice from the center conductor by the distance required to make the ground planes appear infinite at the operating frequencies. If these requirements are met the side walls lie outside of the region of influence of the -R.F. iield and do not appreciably effect the transmission characteristic of the line.

In accordance with one embodiment of the attenuator of the present invention, the ground planes of a doubleground-plane line co'mprise a first pair of opposed side walls of an enclosure and the walls interconnecting the ground planes comprise a second pair of opposed side walls of an enclosure. The enclosure has a pair of end walls `each having a coaxial cable connector centrally located therein. Connected kbetween the center conducto'rs of the two connectors is a cylindrical resistive element shunted at its longitudinal center to the ground planes through a further resistive element. The resistivey elements comprise a T-pad attenuator for dissipating.

energy applied to the doublegroundplane line. The .en-

posed wholly within the enclosure and secured to oneI o'f the side walls of the second pair of side walls, that is, the walls interconnecting the ground planes. The ground planes may be made sufficiently wide that the projection of the pump into the enclosure is not so great that the pump extends into a region where it can affect the RF.. field of the line. The pump must be quite small so as not to extend too far into the enclosure and also' so that it can be placed in the space between the ground planes, the height of this space being dictated by the transmission characteristics of the line. In order to meet the dimensional requirements, the capacity of the pump is limited and a by-'pass is provided around the pump so the pump need only establish a small rapid flow of liquid which will induce flow of the major portion of the liquid through the enclosure, incl-uding the by-pass around the pump.

In a second embodiment of the invention, several attenuators are disposed in a single housing and are cooled by liquid circulation established by a single pump. The characteristic of double-ground-plane lines which permit the pump to be disposed in the enclosure in the first embodiment of the invention also permit several lines to be appropriately located in a single housing without appreciable interaction between the various lines.

It is an object of the present invention to employ a liquid-cooled, double-ground-plane transmission line attenuator at radio frequencies.

It is another object of the present invention to connect a fdouble-ground-plane line attenuator in a coaxial cable for attenuation of R.F. energy.

It is yet another object of the present invention to provide a liquid-cooled, double-ground-plane line attenuator employing resistive elements arranged as a T-pad.

It is still another object of the present invention to provide a double-ground-plane attenuator for connection in a coaxial line, wherein a pump is sealed within the attenuator for circulating a cooling liquid to dissipate heat generated therein.

The above and still further objects, features and `advantages of the present invention will become apparent upon consideratio'n of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

Figure -1 is a cross-sectional view of the side elevation of the attenuator of the present invention;

Figure 2 is a crosssectional View `taken perpendicular to the plane of Figure 1; and

Figure 3 is a schematic showing of a multiple attenuator unit.

Referring now more particularly to Figures `1 and 2 of the accompanying drawings the double-ground-plane line attenuator of the present invention includes a housing generally designated by the reference numeral 1 having a iirst pair of opposed side walls 2 and 3 and a second pair of yopposed side walls 4 and 5 perpendicular to the walls 2 and 3, the walls 2--5 defining a cavity 6 having a rectangular transverse cross-section. The housing 1 yfurther includes a pair of end walls 7 and 8 perpendicular to and integral with the side walls 2 and 3. The end walls 7 and 8 are provided with aligned circular apertures 9 and 10, respectively, which are positioned centrally of the end walls 7 and 8 in the vertical dimension as viewed in Figure l and are positioned somewhat closer to the side wall 4 than the side wall 5 as viewed in Figure 2 of the accompanying drawing The aperture 9 in end wall 7 has a region of reduced diameter adjacent the inner surface of the wall 7 to provide a peripheral flange 11, While the aperture 10 has a region of reduced diameter adjacent the inner surface of `the wall 8 to provide a peripheral flange 12. Circular discs 13 and 14 of dielectric material are disposed respectively in the apertures 9 and 10, the inner surfaces of the discs 13 and 14 being seated against the flanges 11 and 12, respectively, and the outer surfaces being flush with the outer surfaces of the walls 7 and 8, respectively. The discs 13 and 14 function as dielectric windows which permit the passage of R.F. energy therethrough into and out of the housing 1.

The inner surfaces of the side walls 2 and 3 are mirror images of one another about a horizontal center plane passing through the centers of the discs 13 and 14 while the two halves of the inner surface of the side wall 2 between end walls 7 and 8 are mirror images of one another as viewed in Figure l of the accompanying drawings. The surface configuration of the left hand half of the inner surface of the side wall 2 is now described which for the reasons set forth immediately above completely denes the shape of the inner surfaces of the walls 2 and 3. The inner surface of the `left hand half of the wall 2 extends inwardly from the ange 11 lfor a short distance to define a horizontal surface 15 which terminates in a ilat surface 16 sloping toward the center of the enclosure 1 at a constant angle. The surface 16 terminates in a flat surface 17 which extends to within a short distance of the longitudinal center of the housing 1. The surface 17 intersects a vertical surface 18 which extends upwardly for a vertical distance approximately equal to the length -of the vertical projection of the surface 16. The surface 18 terminates in a horizontal surface 19 which extends an equal distance on either side of the horizontal center of the housing 1. The surface 19 4is provided with a rectangular groove 20 for purposes to be described subsequently. The walls 2 and 3 therefore provide a cavity 6 which is a constant height for a short distance inwardly of the end walls 7 and 8 and then decreases uniformly in height as the longitudinal center of the housing 1 is approached. Regions of constant height are again provided between the surfaces 17, and a central portion of greatly increased height is provided by the surfaces 18 and 19 and extends equal distances on both sides of the longitudinal center of the housing 1.

Proceeding with the description of the attenuator, conductive screw 21 extends through the dielectric disc 13 and is connected to a conductive circular disc 22 having one surface abutting the inner surface of the dielectric disc 13. The disc Z2 is connected to a conductive circular disc 23 through a conductive solid cylinder 24. The disc 23 :abuts an open end of a hollow resistive element 25 having a diameter which is small compared to the Width of the side walls 2 and 3. As viewed in Figure 2 of the accompanying drawings the elements Z5 consist of a hollow glass cylinder 26 having a resistive coating 27 disposed on its outer periphery and on its ends. The other end of the resistive element 26 abuts a conductive disc 28, the diameters of the discs 23 and 28 being equal to the diameter of the hollow cylindrical resistive element 26. The conductive disc 28 contacts one surface of a vertical flat, rectangul-ar resistive element 29 which consists of a `glass plate 30' having a resistive coating 31 on its flat, vertical surfaces and on its upper and lower edges. The vertical dimension of the element 29 is such that it extends into the grooves 20 in the side walls 2 and 3 and contacts the bottom of the grooves 20 to provide an electrical contact between the resistive coating 31 and the walls 2 and 3. The horizontal dimension of the -resistive element 29 is determined, in conjunction with the resistivity of the coating 31, by the desired resistance of the element and the heat dissipatingsurface area required. The only specific limitation on the horizontal width of .the element 29 is that it must not contact therwalls 4 and `5 so that cooling medium may be freely circulated in the housing 1.Y Y l The right side of the element 29 is contacted by a conductive disc 32 which abuts one end of a hollow resistive e1ement`33, the other end of which is` connected throughconductive disc 34, conductive cylinder 35 and conductive disc 36 to a conductive screw 37 that extends through the center of the dielectric disc 24. The ele- -ments 32, 33, 34, 35, 36 and 37 are identical to the elements 21, 22, 24, 23, 25, and 26, respectively, and all of these elements are coaxial with an axis passing through the centers of the dielectric discs 13 and 14, this axis also passing through the center of the resistive element 29. The cavity 6 defined by the walls 2--8 of the housing 1 is adapted to be filled with a dielectric liquid such as oil which is circulated Within the housing, by means to be described subsequently, in order to convey heat generated in the resistive elements 25, 29 and 33 to the metallic walls 2 8. The walls 2 and 3 are provided with a plurality of cooling fins 2' and 3', respectively, on their external surfaces in order to facilitate dissipation of the heat in the surrounding air. Although only the walls 2 and 3 are illustrated as having cooling fins it is to be understood that all or any of the walls of the housing 1 may be provided with such members.

The portion of the screw 21 which extends through the dielectric disc 13 threadedly engages a center conductor 38 of a coaxial cable connector 39. The central conductor 38 has a truncated conical portion, the base of which has the same diameter as the elements 25 and 33 and is seated against the dielectric disc 13 and the top of which terminates in a cylindrical portion of such a diameter as to be receivable in the female center conductor of a mating coaxial line connector. An outer conductor 40 of the connector 39 comprises a circular base plate 41 which seats against the end wall 7 of the housing 1 and may be suitably secured thereto as by bolts, not illustrated. Extending horizontally from the plate 41 is a circular portion 42 which terminates in a centrally-disposed, eXternally-threaded, cylindrical member 43 which may be engaged by an internally threaded member of a co-operating coaxial cable connector, not illustrated. The outer conductor 40 is provided with a central aperture 44 which tapers gradually inwardly from the left hand extremity of the connector to a region adjacent the intersection of the conical and cylindrical portions, of the inner conductor 38 and thereafter increases `in diameter to the internal diameter of the flange 11 in the end wall 7. The screw 37 ywhich passes through the dielectric disc 14 in the end wall 8 of the housing lengages a central conductor 45 of a coaxial connector 46 which is substantially identical to the connector 39.

The resistive elements 25 and 33 and the side walls 2 and 3 of the housing 1 constitute a double-ground-plane transmission line wherein the diameters of the elements 2S and 33 are of such a small size with respect to the width of the side walls 2 and 3 that the ground planes constituted by the side walls 2 and 3 appear infinite so far as an RF. field is concerned and leakage of energy is minimized.

Attenuation of the signals applied to the attenuator of the present invention is accomplished by a T-pa section comprising the cylindrical resistive elements 25 and 33 and the flat plate resistive element 29, the elements 25 and 33 constituting the cross arm of the T-pad and the `element 29 constituting the grounded leg thereof. In operation, R.F. energy applied to one of the connectors 39 or 46 and abstracted from the other connector is attenuated by the series resistance of the resistive elements 25 and 33 and by the shunt resistance path to ground through the element 29.

(In order to avoid discontinuities in the transmission paths from a coaxial cable to the attenuator and from the attenuator to a coaxial cable, the inner and outer diameters of the conductors of the coaxial cable must be matched to the spacing between the walls 2 and 3 and the diameter of the resistive elements 25 and 33, respectively. Further, the resistance of the doubleground plane line must be matched to the characteristic impedance of the coaxial lines and the etfects of discontinuities resulting from the passing from an air to a liquid dielectric must be minimized. The conical portion of the center conductors 38 and 45 of the connectors 39 and 46 provide a constant impedance taper which matches the diameter of the coaxial lines to the double-groundplane line. The taper of the internal apertures of the connectors 39 and 46 serve to match the internal diameters of the outer conductors of the coaxial cables to the spacing between the Walls 2 and 3 of the attenuator and the impedance of the attenuator is matched to the characteristic impedance of the coaxial cable by means of a linearly tapering impedance transform provided by the sloping inner surfaces 16 of the side walls 2 and 3. In addition, the eects of the discontinuities resulting in passing from an air to a liquid dielectric are minimized by undercutting the center conductor of the double-groundplane line, the appropriate undercuts being eiected by selecting elements 22 and 24 and 35 and 36 having appropriate diameters with respect to one another and with respect to the elements 25 and 33. Although specific arrangements are illustrated and described for minimizing discontinuities in the transmission paths between the attenuator and associated coaxial lines it is not intended to limit the invention thereto since other well-known techniques may be employed.

The present invention is primarily concerned with providing effective cooling of the electrical energy dissipating elements 25, 29 and 33 of the attenuator so that relatively small resistive elements may be employed to dissipate an appreciable amount of power. It is necessary that the resistive elements be maintained small with respect to the wave length of the operating frequency so that they may be considered as lumped constants and since the attenuator of the present invention is to be employed primarily at R.F. and higher frequencies, the resistive elements are physically small. v

As previously indicated, the housing l1 is filled with oil which is to be circulated within the housing in order to conduct heat from the elements 25, 29 and 33 to the side walls 2-8 of the housing 1. The oil in the housing 1 is circulated by means of a centrifugal .pump 47 secured in a circular aperture 48 in the side wall 5 and disposed wholly within the housing 1. The pump 47 is disposed a considerable distance from the elements 25 and 33 compared to their diameters and consequently lies in a region of low iield intensity so that the conductive and magnetic properties of the lpump elements have substantially no effect upon the R.F. characteristics of the line.

The pump 47 is completely sealed within the housing 1 and comprises a hollow cylindrical housing 49 seated in the aperture 48` in side wall 5. The pump housing 49 includes aperipheral flange 50 adapted to be bolted to the external surface of Wall 5 to secure the pump 47 to the wall 5 and seal the opening 48. The lower end of the hollow pump khousing 49 as viewed in Figure 2 is closed by a thin metallic diaphragm 51 clamped to the lower surface of the ange 50 by means of an annulus 52. In consequence, the pump 47 is completely sealed within the housing 1 and no driving connection `is provided through the walls thereof. Disposed within the hollow housing 47 is a centrifugal impeller 53 mounted on a shaft 54 journaled in bearings 55 and 56. The bearing 55 seals the upper end of the pump housing 47 while the bearing 5.6 is supported on a spider, only one spoke of which, designated by the reference numeral 57, is illustrated in Figure 2. Mounted on the shaft 54 adjacent the diaphragm 51 is a permanent magnet 58, preferably having four poles, which cooperates with a power driven permanent magnet 59 disposed externalh of the diaphragm 51 to rotate the centrifugal impeller 53. The impeller 53 is provided with a plurality of passages 60, each having one end permanently communicating with the central aperture of the pump housing 49 and the other ends selec tively and successively registering, upon rotation of the impeller 53, with one end of a passage 61 in housing 49, the other end of passage 61 communicating with the cavity 6 of housing 1. A second passage 62 in the pump housing 49 extends between the cavity 6 in the housing 1 and the central aperture in the pump housing 49, the passages 61 and 62 being disposed on opposite sides of the plane of the resistive element 29. It is immediately apparent from the drawings that the pump 47 must be quite small to fit into the housing 1 and to fit into the housing 1 without extending too close to the elements 25 and 33, and consequently the capacity of the pump 47 is of necessity small compared to the quantity of oil required to fill the housing 1. To be able to move the oil in suicient quantity and at a high enough rate to effect appreciable cooling of the resistive elements 25, 29 and 33 `an oil by-pass is provided between the end of the element 29 and the pump housing 49 so that the pump 4`7 merely serves to jet the liquid into motion, the body of the liquid moving between element 29 and housing 49. Since the element 29 extends between the side walls 2 and 3 the oil ows in a counter-clockwise path around the element 30 and over the resistive elements 25 and 33. A small portion of the liquid is drawn into the passage 62 and expelled from the passage 61 so as to initiate and maintain movement of the liquid.

The concept of employing the pump merely to jet the main body of liquid into motion permits the use of a pump which may be quite small and therefore may be located wholly within a double-ground-plane line of practical size without interfering with the field patterns of the double-ground-plane line. The ability to seal the pump 47 within the housing 1, eliminates the problems normally encountered in providing oil or other liquid seals about rotating shafts. The pump 47 illustrated in Figure 2 is enlarged relative to its true size to illustrate more clearly its various parts.

It is apparent from the description that the present invention provides an attenuator element for utilization with coaxial transmission lines which element is capable of dissipating large amounts of electrical energy in consequence of the circulation of a cooling liquid over the energy dissipating elements. The concept of employing an attenuator comprising a double-ground-plane transmission line greatly simplifies circulation of a cooling medium over the dissipative elements and eliminates the problems normally encountered in cooled coaxial attenuators resulting from disturbance of the field pattern by ythe liquid entry and exit means. Although the apparatus of the present invention is illustrated and described as a completely sealed unit employing an internally located pump for circulating the cooling liquid, the side Walls 4 and 5 may be provided with input and output connections to an externally located liquid circulation system since the principles of the double-groundplaneline which permit the pump 47 to be located within the housing 1 without disturbing the R.F. field pattern also permit input and output connections to be disposed in the walls 4 and 5 wihout effecting the field pattern.

The embodiment of the invention illustrated in Figures 1 and 2 of the accompanying drawing provides only a single T-pad attenuator per housing which due to the fixed nature of the resistive elements 25, 29 and 33 restricts the utilization of each apparatus to occasions where its specific characteristics are required. ln order to extend the range of applicability of each apparatus, each housing is modified to include plural T-pad attenuator units cooled by liquid circulated by a single pump.

Refering now more particularly to Figure 3 of the ac- Companying drawings, wherein prime members designate elements corresponding to identical elements of Figures l and 2 having the same basic number, a housing 63 has end walls 7 and S and side walls 3', 4 and 5', the fourth side wall not being illustrated. Three T-pad attenuator units 64, 65 and 66 are arranged side-by-side between side walls 4 and 5, each attenuator unit comprising resistive elements 25', Z9 and 33'. A single pump 47 is employed to circulate oil over the three units 64,- 65 and 66 and each of the units is designed to produce a different attenuation.

`lt is possible to locate plural double-ground-plane line attenuator units within a single housing so long as the spacing between the units is such that each unit lies outside of the limits of the RP. field produced by its adjacent lines and vice versa. The characteristics of a double-ground-plane line which permits a pump to be disposed relatively close thereto also permits plural attenuator `units to be disposed side-by-side without disturbing the electrical properties thereof. Although the apparatus is described as providing three units in a single housing, it is not intended to restrict the invention to any specific number.

While l have described and illustrated one specific embodiment of my invention, it will be clear that variations of the general arrangement and of the details of construction which are specifically illustrated and de- 'scribed may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

l. A double-groundplaneline attenuator adapted to be connected in series between two coaxial cables, said attenuator comprising an enclosure having a first pair and a second pair of opposed metallic side walls and a pair of end walls, at least one input terminal means disposed in one of said end walls and at least one output terminal means disposed in the other of said end walls, first resistive means connected in series between said terminal means, the relative widths of said side walls of said first pair of side walls and said first resistive means being such as to provide a double-ground-plane transmission line, second resistive means positioned at right angles to said first resistive means and said side walls and electrically connected between said first resistive meams and the side walls of said first pair of side walls, said enclosure being filled with a dielectric uid and means disposed substantially wholly within said enclosure for circulating said dielectric fiuid, said last-mentioned means being disposed such that it does not affect the field pattern of said attentuator.

2. A double-ground-plane-line attenuator for insertion m a coaxial line, said attenuator comprising, an enclosure having a rectangular cross-section, said enclosure including a first pair and a second pair of opposed metallic side walls and a pair of opposed end walls, an input terminal means disposed in one of said end walls and an output terminal means disposed in the other of said end wails, first resistive means connected in Series between said terminal means, the relative widths of said first re- SlStive means and said side walls of said first Ipair of side 'walls being such as to provide a double-ground-plane transmission line, a fiat rectangular resistive means disposed perpendicular to said side walls, means for electrically and physically connecting said rectangular resistive means between the longitudinal centers of said first resistive means and said side walls of said first pair of side walls, said enclosure being filled with a liquid dielectric and means disposed substantially wholly within said enclosure secured to at least one of said side walls of said second pair of walls for circulating said liquid dielectric, said last-mentioned means being disposed such that it does not effect the field pattern of said attenuator.

3. A double-ground-plane-line attenuator for insertion in a coaxial line, said attenuator comprising an enclosure having a rectangular transverse cross-sectional configuration, said enclosure including a first pair and a second pair of opposed side walls and a pair of opposed end walls, a flat rectangular resistive element disposed equidistant between said end walls and perpendicular to said side walls, said resistive element physically and electrically contacting said side walls of said first pair of side walls, an input terminal means adapted to be connected to a coaxial cable and disposed in one of said end walls, an output terminal means adapted to be connected to a coaxial cable and disposed in the other of said end walls, a first hollow cylindrical resistive element centered between said side walls of said first pair and extending between said input terminal means and said rectangular resistive element, a second hollow cylindrical resistive element centered between said side walls of said first pair and extending between said rectangular resistive element and said output terminal means, the diameter of said cylindrical elements being equal and being such with respect to the width of and spacing between said side walls of said first pair of side walls as to provide a doublegroundplane transmission line, said enclosure being filled with a liquid dielectric and a pump disposed within said enclosure and secured to at least one of said side walls of said second pair of side walls for circulating said liquid dielectric, the depth of penetration of said pump into said enclosure being sufficiently small relative to the characteristics of said transmission line as to be electrically isolated from said transmission line.

4. The combination in accordance with claim 3, wherein said terminal means include constant impedance taper means for matching the size of the central conductors of the coaxial cables to the size of the cylindrical resistive elements of said attenuator.

5. The combination in accordance with claim 3, further comprising means for matching the impedance of said rectangular resistive element to the impedance of a coaxial cable to which said attenuator is to be connected, said matching means including tapered inner surfaces of said side walls of said first pair of side walls.

6. The combination in accordance with claim 3, wherein said means for circulating comprises a pump located wholly within said enclosure, and permanent magnet means for coupling said pump to a source of rotary motion located externally of said enclosure.

7. The combination in accordance with claim 6, wherein said pump imparts a high velocity to a quantity of dielectric liquid which is small compared with the total quantity of liquid dielectric in said enclosure and means for locating said pump within said enclosure such that a liquid by-pass region is provided around said pump.

8. A double-ground-plane-line attenuator adapted to be inserted in a coaxial cable comprising an enclosure having a first pair and a second pair/of opposed side walls and a pair of end walls, a rplurality of input terminal means disposed in one of said end Walls, a plurality of output terminal means disposed in the other of said end walls, each of said input terminal means lying along an axis parallel to said side Walls and passing through a different one of said output terminal means, a plurality of first resistive elements, each connected in series between aligned input and output terminal means, the relative widths of each of said first resistive means and said side walls of said first pair of side walls being such as to provide a plurality of electrically independent doubleground-plane transmission lines, a plurality of second resistive means disposed at right angles to said side walls and said rst resistive means, each of said second resistive means being electrically connected between the longitudinal center of a different one of said first resistive means and the longitudinal centers of said side walls of said rst pair of side walls, said enclosure being filled with a liquid dielectric and means disposed substantially wholly within said enclosure secured to one of said side walls of said second pair of side walls for circulating said liquid dielectric, said last-mentioned means being disposed such that it does not alect the fleld pattern of said attentuatcr.

References Cited in the tile of this patent UNITED STATES PATENTS 2,534,437 Ginzton Dec. 19, 1950 2,620,396 Johnson et al. Dec. 2, 1952 2,752,572 Bird et al. June 26, 1956 2,859,020 Eddy Nov. 4, 1958 FOREIGN PATENTS 826,472 Germany Ian. 3, 1952 495,610 Canada Aug. 25, 1953 

